The prompt-turn finally ran trim_memory (gc.collect + malloc_trim, 20-50 s
on multi-GB heaps), the thinking-sound stop and the TTS sentinel BEFORE
marking the session idle and logging the "tui turn finished" bookend. A
slow or blocked trim therefore kept the session busy and the bookend
missing for its whole duration.
Split _finish_turn into _release_turn_scopes (snapshots, one-turn model
restore, approval/secret/terminal/session scopes — unchanged order and
logging) and _post_turn_housekeeping (trim, thinking sound, TTS sentinel,
then the HERMES_HOME override reset so the trim still reads the session
profile's config, #58576). The settle block runs between them verbatim.
Refs #131740 (the reported 600 s watchdog hang is inside
agent.run_conversation, so this hardening does not close it).
Salvaged from #131773.
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